Staged combustion systems are used to improve combustion by introducing successive portions of fuel into the combustion process to allow the oxidant and fuel to react in multiple zones or stages. This produces lower peak flame temperatures and other favorable combustion conditions that reduce the generation of nitrogen oxides (NOx). A wide variety of staged combustion methods are known and used in combustion applications including process heaters, furnaces, steam boilers, gas turbine combustors, coal-fired power generation units, and many other combustion systems in the metallurgical and chemical process industries.
The combustion of a gaseous fuel with oxygen in an oxygen-containing gas such as air occurs when a fuel-oxygen-inert gas mixture having a composition in the combustible region reaches its autoignition temperature or is ignited by a separate ignition source. When the combustion occurs in a three-dimensional process space such as a furnace, the degree of mixing is another important variable in the combustion process. The degree of mixing in the furnace, especially in the regions near the burners, affects localized gas compositions and temperatures, and therefore is an important factor in operating stability.
In combustion processes, particularly in staged combustion processes for NOx reduction, it is important to have good flame stability and proper location of the flame front relative to the points at which staging fuel is introduced into the combustion space. In conventional combustion systems, flame stability may be maintained by the use of fuel injection devices and internal recirculation patterns to improve the contact of the fuel stream with the combustion atmosphere and to provide the ignition energy required to sustain flame stability. Improper control of flame stability and flame location in staged combustion systems, particularly during cold startup, process upsets, or turndown conditions, may result in undesirable combustion performance, higher NOx emissions, and/or unburned fuel. This latter condition could lead to substantial pockets of fuel in the furnace and the possibility of an uncontrolled energy release.
There is a need in staged combustion processes for improved flame stability and complete fuel combustion, particularly during unsteady-state operating periods such as cold startup, process upsets, or process turndown conditions. Improved staged combustion systems to meet these needs are disclosed by embodiments of the present invention described below and defined by the claims that follow.